Two-stroke engines include an exhaust port in the cylinder wall, which is opened and closed as the piston reciprocates in the cylinder. It has been recognized that in multiple cylinder two-stroke engines reflected pressure fluctuations under idle or low speed conditions can adversely effect the combustion. At low or idle speeds, the pressure waves are not normally tuned so that reflected pressure waves from other cylinders in the bank, or from the same cylinder, can have an adverse effect on the engine combustion.
In view of this, it has been proposed to utilize a butterfly valve in the exhaust port of a two-stroke motorcycle engine and control the operation of the valve through the intake manifold vacuum as disclosed in "Emission Control Of Two-Stroke Motor Cycle Engines By The Butterfly Exhaust Valve", Society of Automotive Engineers Inc., Sept. 8-11, 1980.
U.S. Pat. application Ser. No. 07/064,195, filed June 19, 1987, also discloses an exhaust valve throttling mechanism for a two-stroke engine which can be incorporated with a multiple cylinder engine, such as a V-6 marine engine employing two banks of three in-line cylinders. In accordance with the invention of that patent application, the engine block is provided with a pair of openings or passages, each of which intersects the exhaust ports of a bank of cylinders and a shaft is mounted for rotation with each passage and carries a plurality of shutters or valve members, each of which is mounted within one of the exhaust ports. Advancement of the throttle will operate through a linkage to rotate the shafts to move the valves toward the open position. Conversely, a reduction of engine speed will act to move the valve members toward the closed position where the valve members will act to substantially block or restrict the propagation of pressure waves back to the cylinders at low and idle speeds.